1. Field of the Invention
The present invention relates to a control method, control circuit, and control program for a power retractable mirror which allow motor operation to be stopped (allow a motor to be turned off) reliably when a mirror rotator is mechanically locked at a driven end of the mirror rotator.
2. Description of the Related Art
Power retractable mirrors are used as vehicle outer mirrors (mainly side mirrors), in which a mirror rotator (including a mirror housing and part which rotates together with the mirror housing) is moved by a motor from working (standing) position to retracted position and from retracted position to working position. Normally, a rotational movement of the power retractable mirror is started when the motor is started with the throw of a switch in the vehicle. Once the motor is started, the mirror rotator rotates automatically from standing position to retracted position (or from retracted position to standing position). When the rotational movement is mechanically locked at a driven end (retracted position or standing position) by a stopper, the motor drive stops automatically.
Techniques for automatically stopping the motor drive at the driven end of the rotational movement of the mirror rotator include those described in Japanese Patent Laid-Open No. 8-40146 and Japanese Patent Laid-Open No. 2002-347522. The technique described in Japanese Patent Laid-Open No. 8-40146 involves inserting a resistor in a drive circuit of the motor and stopping the motor by detecting a voltage across the resistor. Specifically, when the rotational movement of the mirror rotator is mechanically locked at its driven end, a current flowing through the motor increases (a locked-rotor current flows), causing an increase in the voltage across the resistor, and the motor is stopped when the increase in the voltage is detected. The technique described in Japanese Patent Laid-Open No. 2002-347522 involves inserting a PTC element (positive temperature coefficient thermistor) in a drive circuit of the motor and detecting a voltage at one end of the PTC element. Specifically, when the rotational movement of the mirror rotator is mechanically locked at its driven end, a current flowing through the motor increases, causing the PTC element to generate heat, which in turn causes an increase in resistance of the PTC element thereby causing an increase in the voltage at the end of the PTC element, and the motor is stopped when the increase in the voltage is detected.
With the technique described in Japanese Patent Laid-Open No. 8-40146, if there is a small difference between locked-rotor current and steady-state current, it is difficult to detect mechanical locking of the stopper. If the mechanical locking fails to be detected, the motor continues to draw power, which can result in burnout.
The technique described in Japanese Patent Laid-Open No. 2002-347522 has the advantage that it is easier to detect mechanical locking than with the technique described in Japanese Patent Laid-Open No. 8-40146 even if there is a small difference between locked-rotor current and steady-state current. However, even with the technique described in Japanese Patent Laid-Open No. 2002-347522, if the control circuit including the PTC element is placed outside the mirror (especially in the vehicle), it is difficult to detect mechanical locking of the stopper due to temperature differences between placement location of the mirror (outside the vehicle) and placement location of the control circuit (inside the vehicle). That is, PTC elements have short detection time at high temperatures and long detection time at low temperatures, where the detection time is the interval between the time when mechanical locking occurs and the time when detection temperature is reached. Therefore, if mechanical locking occurs in summertime when it is hot outside the vehicle (the motor inside the mirror is hot) and it is cool inside the vehicle (the PTC element is cold) due to air conditioning, the PTC element is slow to detect the mechanical locking even though the motor is liable to generate heat, which can result in motor burnout.